Lead for implantable cardiac prosthesis, including protection against the thermal effects of mri fields

ABSTRACT

A heat dissipating housing to be positioned at a distal end of a stimulation/defibrillation lead, the housing including a housing body having a distal contact face at a distal end, an inner wall, and an outer wall. The housing further includes an anchoring screw, wherein at least a portion of the anchoring screw extends axially from the distal end of the housing body. The anchoring screw is configured to secure the distal contact face in contact with tissue of a patient to form a thermal bridge between the tissue and the housing body. The housing body is electrically insulated and thermally conductive to dissipate heat transferred through the thermal bridge to the outer wall of the housing body.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a continuation of U.S. application Ser. No.13/545,259 entitled “Lead for Implantable Cardiac Prosthesis, includingProtection Against the Thermal Effects of MRI Fields,” filed Jul. 10,2012, which claims the benefit of French application Ser. No. 11/56360entitled “Lead for implantable cardiac prosthesis, comprising means forprotection against the thermal effects of MRI fields” and filed Jul. 12,2011, both of which are hereby incorporated by reference in theirentireties.

FIELD OF THE INVENTION

The present invention relates to “active implantable medical devices” asdefined by the Jun. 20, 1990 Directive 90/385/EEC of the Council of theEuropean Communities, which includes devices that continuously monitor apatient's heart rhythm and deliver to the heart, if and as necessary,electrical stimulation pulses, for pacing, resynchronization,cardioversion and/or defibrillation, and more particularly to leads forintracardiac stimulation or defibrillation that collect (detect)depolarization signals from the patient's heart, which leads areconnected to an implantable device generator, and to techniques forprotecting the leads when the patient must be subjected to examinationby magnetic resonance imaging (MRI).

It is known to have intracardiac leads provided at their distal end witha screw for anchoring the distal head of the lead in the tissue of theendocardium so that an electrode of the lead makes electrical contactwith the patient's myocardium tissue. In addition, in the case of a leadwith an “active” screw, once in place the screw itself acts as a distalelectrode for detection/stimulation of the myocardium. One such screwlead, with a retractable screw, is disclosed by EP0591053A and itscounterpart U.S. Pat. No. 5,447,534 (both assigned to Sorin CRM S.A.S,previously known as ELA Medical), which describes a type of lead ismarketed under the brand name STELIX (registered trademark) by SorinCRM, Clamart, France.

These leads can be endocardial leads (i.e., placed in a cavity of themyocardium in contact with the wall thereof), epicardial leads (i.e.,placed on the outside of the heart, in particular to define a referencepotential, or to apply a shock), or intravascular leads (i.e.,introduced into the coronary sinus to a location facing, e.g., the leftventricle wall).

The present invention is however applicable to other types of leads, forexample, those leads having a distal electrode that remains on thesurface of the wall of the patient's tissue to be detected/stimulated,and which are then provided with anchoring tines to maintain them inplace at the chosen site. EP 0784993A1 (and its counterpart U.S. Pat.No. 5,800,499) and EP077908.QA1 (all assigned to Sorin CRM S.A.S.,previously known as ELA Medical) describe examples of such tined leads,which are incorporated herein by reference.

MRI examinations are presently contraindicated for patients with animplanted pacemaker or defibrillator. This is because of severalproblems caused by MRI:

-   -   Heating close to the electrodes of the lead connected to the        generator;    -   Attraction forces and torques exerted on the device while        immersed in the very high static magnetic field of the MRI        device; and    -   Unpredictable behavior of the device itself, due to exposure to        these extreme magnetic fields.

The present invention aims to solve the first problem type. The heatingproblem appears especially in the vicinity of electrodes mounted at thedistal end of the leads. Indeed, leads placed in the MRI imager act likeantennas and pick up the radio frequency field (RF) emitted by theimager. Induced currents circulate in the conductors of the leadsimmersed in the RF field, causing heating of electrodes in contact withthe blood and consequently heating of surrounding tissue. The heating atthe electrodes is proportional to the density of current flowing thereinand the smaller the surface of the electrode (the typical case being thesurface of an active screw), the higher the current density andtherefore the greater the heating of the surrounding tissue.

In practice, depending on the configuration of the generator, the leadsand the MRI imaging, the temperature rise observed experimentallytypically ranges from 8° C. (carbon electrodes) to 12° C. (metalelectrodes), and sometimes even up to 30° C.

But the temperature increase should not exceed what is specified in theindustry standard EN 45502-1 and its derivatives, which is less than 2°C. Indeed, an increase of 4° C. can cause a local cell death that has animmediate effect, among others, to substantially and irreversibly changethe detection and stimulation thresholds, or even lead to complete lossof capture of the patient's heart beat.

It is certainly possible, as described in particular in U.S. Pat.Publication No. 2003/0204217 A1 and U.S. Pat. Publication No.2007/0255332 A1, to provide an “MRI” safety mode in which a protectioncircuit connects to ground all conductors to prevent the flow ofparasitic induced currents exposed to an MRI field. But this approachprevents the device from remaining functional for the duration of theMRI examination, which may last several minutes. It is therefore highlydesirable that the implanted device can continue to provide seamlessdetection of depolarization potentials and possible delivery ofstimulation pulses to the myocardium during an MRI examination.

To reduce the induced currents without disconnecting (i.e., opencircuiting) or connecting to ground (i.e., grounding) the conductors,various techniques have been proposed, based primarily on putting inseries with the conductor an impedance opposing current flow in an MRIexamination situation. It may be a single coil (see, e.g., U.S. Pat.Publication No. 7,123,013 B2), or a resonant tank circuit tuned to theRF frequency of the imager (see, e.g., U.S. Pat. Publication No.2011/0106231, U.S. Pat. Publication No. 2010/0208397 A1 and U.S. Pat.Publication No. 2011/0054582 A1).

A passive protection circuit consisting in placing a PIN diode inparallel with a resistor in series with the electrode also has beenproposed (cf. U.S. Pat. Publication No. 2008/0154348 A1). Yet anotherapproach, disclosed in EP 2198917 A1 and its counterpart U.S. Pat.Publication No. 2010/0160989 (both assigned to Sorin CRM S.A.S.,previously known as ELA Medical) is, in the case of a bipolar lead, todisconnect one of the conductors from normal functioning and to connectit to the ground of the housing of the generator, so that this conductorcan act as protection shielding for the other conductor, which remainsfunctional.

These options have, however, a number of limitations, including:

-   -   The systems implementing the commutations for their activation        require a magnetic field detector in the generator;    -   Passive protection tank circuits are calculated for a specific        imager frequency;    -   Integrated protection circuits may require an additional        conductor in the lead and, consequently, cannot be used with any        generator without suitable hardware modification of its        circuits;    -   The protection components, such as diodes, are likely to cause        substantial loss of energy when delivering stimulation pulses        from the generator to the electrodes;    -   The addition of an integrated protection circuit in the distal        portion of the lead body can cause, when the device is immersed        in a MRI field, the reflection of an RF voltage to the generator        that must be filtered and dissipated;    -   The incorporation of a protection circuit for self-protection of        the lead in all circumstances is extremely sensitive in terms of        technology, given the physical constraints: e.g., external        diameter is limited to 5 French (1F=1/3 mm), a need for an        internal lumen in the lead body, and limitation of the length of        the rigid part in the lead head; and    -   The continuing high cost of implementation of these        technologies.

Another disadvantage is that the protection circuit itself may undergo arise in temperature that is transmitted to the proximal electrode andthus to the tissue of the heart wall.

Thus, in the U.S. Pat. Publication No. 2011/0106231 A1 above, theresonant tank circuit, which blocks the current flow to the anchoringscrew forming an active electrode, comprises an inductor housed in thelead head. When the assembly is placed in a MRI field at a frequencycorresponding to that of the tank circuit, high current flows in theinductor and causes significant heating within the lead head. Thedocument proposes to evacuate this heat to the outside by equipping thelead head with a liner in the outer region of the inductance, forming aheat radiator. The heat produced in the inductance is then distributedradially in the lead head and then through the liner, to transfer theheat to the surrounding blood flow.

It is emphasized that in this structure the thermal diffuser is placedin line with the inductor (that is to say at about the middle of theterminal part of the lead head), and that the distal end of the lead isnot affected by the thermal diffuser. Specifically, the distal end ismade of a flexible material such as silicone which limits the contactpressure on the tissues. Silicone is a poor conductor of heat, but thisis not a problem because in the presence of an RF field the screw is nolonger powered (due to the tank circuit) and the tissues are not warmingup at this level.

It should be understood that the structure described in this documentrequires the presence of a relatively large inductance, and electricalisolation between the inductor (which is connected in series with thestimulation conductor) and the thermal diffuser (which is necessarily incontact with the surrounding blood medium). All these constraintsincrease the complexity of implementation and the overall volume of thelead head.

U.S. Publication No. 2010/0208397 A1 and U.S. Pat. Publication No.2011/0054582 A1 described above, disclose comparable lead headconfigurations, with a tank circuit to prevent against the harmfuleffects of an MRI field, and a heat diffuser arranged in line with theinductance of the tank circuit, so as to allow transfer of the heatgenerated in the inductor in the radial direction to the surroundingblood medium.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a new leadconfiguration having a permanent “auto-protection” of the lead againstthe deleterious thermal effects of MRI fields without use of anyprotection circuitry or additional electronic component.

It is another object of the present invention to provide a screw leadconfiguration that, in all circumstances, significantly reduces thetemperature rise at the distal end of the lead during exposure to an MRIfield and avoids the partial or total destruction of tissue around theanchoring screw.

The starting point of the present invention is the discovery that theanchoring screw and its deployment system (in the case of a retractablescrew) are encapsulated in a mechanism whose outer walls areelectrically insulated, made of a material such as polycarbonate,polyurethane, silicone or polyetheretherketone (“PEEK”). One commonelement in all these materials is their low thermal conductivity. Themechanism containing the screw and its activation system, generallyreferred to as the “housing”, is not only an electrical insulator (whichis essential) but also a thermal insulator. The result is that in caseof heating, the heat flow is confined to the center of the screw, withlittle opportunity to spread (diffuse) outward.

Indeed, the heat exchange between this zone and the adjacent volumes arelimited:

-   -   On the distal side (i.e., at the tip of the screw) and on the        radial side (i.e., on the periphery of the tip of the screw),        the low thermal conductivity of muscle tissue in which the screw        is implanted, and    -   On the proximal side (i.e., at the side of the housing in        contact with the cardiac wall, but outside thereof), by the        effect of a “thermal cap” attributable to the low conductivity        of the material of the housing.

Thus, upon heating of the screw under the effect of an intense MRIfield, the temperature rise is even more pronounced when it is notpossible to transfer the heat generated to the outside: neither to themass of the myocardial wall (due to low thermal conductivity) or to thesurrounding blood flow (due to the screening caused by the thermal capeffect of the housing enclosing the screw and deployment mechanism).

The present invention seeks to solve these problems by providing the endof the lead, on the housing enclosing the anchoring screw and itsdeployment mechanism, with an improved heat sink, preferably one ascontinuous a mass as possible, between the outer surface of the housingand the core of the screw anchored in muscle tissue (wherein thelocalized heating occurs), in order to utilize the heat dissipationeffect generated by the blood flow around the housing of the lead head.The heat flow generated at the screw is then mainly dissipated in theproximal direction, into the blood stream via the thermally conductivehousing, thus limiting the rise in temperature at the tissue level.

Broadly, the present invention is directed to a lead for intracardiacstimulation or defibrillation, known, for example, from U.S. Pat.Publication No. 2011/0106231 A1 mentioned above, which is incorporatedherein by reference, comprising a tubular flexible sheath terminated atits distal end by a lead head, the lead head comprising an at leastpartially electrically insulating tubular outer housing, a means foranchoring the lead head, connected to the tubular housing, a distalstimulation electrode, arranged at the end of the lead head, and a heatsink member, the heat sink member being electrically isolated andcarried by the tubular housing in an outer region thereof and comprisinga solid part made of a thermally conductive material.

In one embodiment, the solid part of the heat sink element extends inthe axial direction to the distal end of the lead head, and isterminated by a contact end face adapted to come into contact with awall of a muscle of the patient to form a thermal bridge for heattransfer between this wall and the solid part.

In a preferred embodiment, the lead is a screw lead, and the means foranchoring comprises a projecting helical anchoring screw extendingaxially along the tubular housing. More preferably, the screw is anactive screw electrically conductive on at least one end portion,forming said stimulation electrode. In this latter case, it may beadvantageous to provide the screw with a spiral element, made of athermally conductive material, inserted between the turns of the helicalanchoring screw so as to fill the space existing between these turns.

The present invention is also applicable to leads wherein the means foranchoring includes one or more tines radially projecting to the outsideof the tubular housing, and the stimulation electrode is a distalelectrode to be supported against a muscle wall.

In a preferred embodiment, advantageously, the front face of the distalelectrode contact is a radial plane face forming the distal end of thetubular housing.

In a lead configuration in which the outer housing comprises a tubularinner housing to a proximal end of the means for anchoring, preferablythe solid part of the heat sink element extends in the radial directionwith a heat transfer continuity solution from the housing to the outerfree wall of the housing. In a preferred embodiment, it is advantageousto provide a thermally conductive material filling the cavity, such as agel, including rehydratable hydrogel.

In one embodiment, the thermally conductive material of the solid parttypically has a thermal conductivity of at least 5 W/mK.

In one embodiment, the thermally conductive material of the solid partis preferably a radio-transparent material. It can in particular be ametallic material such as titanium or a titanium alloy.

In one embodiment, the solid part has on its surface an electricallyinsulating coating, preferably a thermally conductive material, such asa diamond deposition.

In the case of a screw lead, the screw also may be provided on a part ofits length an electrically insulating coating of a thermally conductivematerial, such as a diamond deposition.

In a certain exemplary embodiment there is a heat dissipating housing tobe positioned at a distal end of a stimulation/defibrillation lead. Thehousing includes a housing body having a distal contact face at a distalend, an inner wall, and an outer wall and an anchoring screw, wherein atleast a portion of the anchoring screw extends axially from the distalend of the housing body. The anchoring screw is configured to secure thedistal contact face in contact with tissue of a patient to form athermal bridge between the tissue and the housing body. The housing bodyis electrically insulated and thermally conductive to dissipate heattransferred through the thermal bridge to the outer wall of the housingbody.

In another exemplary embodiment there is a heat dissipating housing tobe positioned a distal end of a stimulation/defibrillation lead,including a housing body having a distal contact face at a distal end,an inner wall, and an outer wall, and an electrode positioned near adistal portion of the housing. The housing further includes an anchoringmechanism extending from the distal end of the housing body, to anchorthe distal contact face to tissue of a patient and a first and a secondthermal bridge for transferring heat from the tissue to the housing. Thefirst thermal bridge is formed between the tissue and the distal contactface of the housing body and the second thermal bridge is formed betweenthe anchoring mechanism and inner wall of the housing body.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, characteristics and advantages of the presentinvention will become apparent to a person of ordinary skill in the artfrom the following detailed description of preferred embodiments of thepresent invention, made with reference to the accompanying drawings, inwhich like reference characters refer to like elements, and in which:

FIG. 1 illustrates the head of a lead of the present invention, inlongitudinal section through an axial plane; and.

FIG. 2 is an enlarged view of the right side of FIG. 1, showing themethod for the heat transfer within the distal end of the lead incontact with the heart wall.

DETAILED DESCRIPTION

With reference to the drawings FIGS. 1-2, embodiments of devices inaccordance with the present invention will now be described.

With reference to FIG. 1, a lead head 10 of a retractable screw lead isillustrated in a situation with the anchoring screw 20 in the deployedor extended position, anchored in the tissues of the heart wall. Leadhead 10 is mounted at the end of a sheath 12 with which it constitutesthe lead body. Sheath 12 has the form of a flexible hollow tubeincorporating two (or more) electrical conductors 14, 16, connected torespective electrodes. Conductor 14 is connected to a proximal ringelectrode 18, and conductor 16 is connected to anchoring screw 20, bymeans of a mobile body 22 having at its proximal end a tail 24 that iselectrically and mechanically connected to conductor 16 so as to ensureelectrical continuity from conductor 16 to screw 20 located at thedistal end.

Lead head 10 also includes a mechanism (not shown in detail) fordeployment of screw 20 so that, when deployed, screw 20 can be anchoredin wall 26 of the endocardium, to ensure a mechanical connection withthe myocardial tissue and prevent the displacement or dislodgement oflead head 10 once it is anchored in place. Screw 20 is in thisembodiment an “active screw” meaning that it is electrically conductiveand acts as the distal sensing/pacing (also calleddetection/stimulation) electrode by its connection to the generator viaconductor 16.

Screw 20 and its deployment mechanism are housed in a rigid element 28of lead head 10, generally referred to as a “housing” or a “can”, oftubular shape.

To increase the efficiency of the screw, the latter is made conductiveon the end portion of its distal end in the vicinity of the tip, with areduced contact surface, typically of the order of 2 mm².

During an MRI exposure, the RF waves sensed by the lead body create acurrent flow that in turn causes a temperature rise at the distal end,particularly in the region 30 in the center of the conductive portion ofthe distal end of screw 20. Given the reduced contact surface, thecurrent density at this location is particularly high, causingsignificant local heating of tissues which, if prolonged, may result inpartial or total destruction of cells, as explained above. This heatingis symbolized by the dashed lines 32 of the central region 30 of thescrew.

The present invention proposes to diffuse and remove the heat thusgenerated, by providing the housing 28, at least in its most distalpart, with a solid part 34 made of a thermally conductive material, soas to transfer (i.e., thermally conduct) to the proximal region the heatgenerated in region 30, to diffuse the heat in the distal area indicatedby the region outlined 36 and to discharge it into the blood flow(volume 38) surrounding solid part 34.

It should be understood that solid part 34 is a unitary memberconstituting the case or housing of the distal region of the lead, witha thermal continuity solution between the muscle contact face 44 and thecylindrical outer surface immersed in the flow blood 38, or between theinner surface of the housing of the anchoring screw and the samecylindrical surface immersed in blood flow 38. It is indeed importantthat the heat transmission takes place in a way that is not interruptedby a thermal barrier material, whether made of air or an electricalinsulator.

By transferring the heat away from region 30 where it arises, thetemperature rise is limited in region 30 and in the surroundingmyocardium tissue.

FIG. 2 is an enlarged view of the right side of FIG. 1, showing themethod for the heat transfer within the distal end of the lead. In thisfigure, reference 44 designates the front end surface of solid part 34,which is preferably a circular flat surface coming in contact with wall26 of the endocardium. The typical dimensions of the support surface areof the order of 5 to 7 French (1.66 to 2.33 mm) for the outside diameterOD and 1.2 to 2 mm for the inside diameter ID. The heat generated inregion 30 at the center of the conductive portion of the distal end ofscrew 20 is transferred to solid part 34 via this contact face 44, whichacts as a thermal bridge between the endocardium 26 and solid part 34.

The heat thus transfers essentially in the axial direction through theinterface between contact face 44 and endocardium 26 (arrows 46), and inthe mass of solid part 34 (arrows 48), which in turn transfers the heatto volume 38 and the surrounding blood flow (arrows 50).

The heat transfer from region 30 where the heat arises to the volume ofblood flow 36 is also made by the intermediary of screw 20 itself,through the coils thereof that are in contact with the inner wall ofcylindrical solid part 34 (arrows 52).

Thus, from the heat conduction point of view, solid part 34 is placed incommunication with region 30 via two separate thermal bridges, (i)support surface 44 and (ii) anchoring screw 20.

To promote heat transfer through anchoring screw 20, it is advantageousto provide a thermally conductive spiral element 54 located between theturns of screw 20, so as to obtain a set with adjacent turns. Thisadditional spiral element 54 has the function of filling the empty spacebetween the turns of the screw 20 and thus to increase the mass ofmaterial capable of transferring heat to solid part 34. It may be madeof titanium or, ideally, platinum (to benefit from the much higherthermal conductivity of that material).

Also advantageously, a thermal conductive gel may be introduced into thevolume between anchoring screw 20 and the inner diameter of solid part34 (e.g., the empty space referenced 56 in FIG. 2). The presence of sucha thermally conductive gel eliminates any air which is highly resistantto heat transfer between the anchoring screw and the inner wall of solidpart 34, in order to provide, here again, optimum heat transfer in theradial direction. The thermal conductive gel is preferably a particularhydrogel applied before implantation in a dry form, such that itinstantly rehydrates on contact with water or blood. The hydrogel may bea particular polyvinylpyrrolidone (PVP) gel, which has, in the hydratedform, thermal characteristics near water (which is its main constituent,in this form). The other advantage of PVP gel is its well documentedbiocompatibility.

The choice of material for the thermally conductive solid portion 34needs to take into account levels of typical thermal conductivity ofdifferent materials that may be encountered or used. This parameter isshown in Table 1 below for a variety of known materials.

TABLE 1 Material Thermal Conductivity (W/mK) Air 0.03 Silicone 0.11Parylene 0.14 PEEK 0.25 Blood 0.50 Muscle 0.54 Water 0.60 Carbon 4 to 6Titanium 7.50 Stainless steel 26.00 Platinium 71.60 Diamond 1000 to 2600

The materials normally used so far for producing housing 34 of the leadhead, such as silicone, parylene, PEEK, all have a very low thermalconductivity, much less than 1, which produces an effect of preventingthe thermal barrier diffusion of heat generated during an MRI at theactive part of the screw tip.

To negate this thermal barrier effect, and replace it with a thermallyconductive pumping effect or heat sink, the invention proposes to selecta material having a much higher thermal conductivity, typically at leastabout 5.

Given the biocompatibility constraints, it is possible to choose forsolid part 34, in place of silicone or PEEK, a metal such as titanium,which has the advantage of being radio-transparent, and thereforepreserves the functionality of other radiopaque markers enabling thesurgeon to control the deployment of the screw under fluoroscopy.However, it is necessary to electrically insulate the outer surface ofthe solid part 34 made of titanium.

In this regard, it is known to isolate titanium parts by applying acoating of a material such as parylene, but the table above indicatesthat the thermal conductivity of this material is very low. To maintainthe efficiency of the thermal bridge, therefore, the present inventionproposes to replace the conventional parylene coating by a surfacedeposit of a material with high thermal conductivity, in particular by adiamond deposit. This material is particularly interesting in thisapplication because it combines a very high thermal conductivity(1000-2600) and a satisfactory electrical isolation capacity. Thedeposit of a diamond coating on a metal cylinder is a technology initself known, but it had so far been used mainly to benefit from theproperties of very low friction coefficient of diamond, and thusfacilitate sliding during introduction, but had never been proposed inthe context of the present invention to take advantage of itsexceptionally high thermal conductivity.

With reference to FIG. 1, insulating coating 40 illustrated as coatingover the entire length of the solid part 34 with the possible exceptionof a region housing a collar 42 for steroid elution, near the area ofcontact with cardiac wall 26.

The diamond coating can be achieved not only on the solid and thermallyconductive part of solid part 34 enclosing the screw 28 and itsdeployment mechanism, but also on the screw itself, except of course foran uncoated surface at the end of the tip to maintain the basicelectrically conductive function of an active screw (substantiallycorresponding to the last two millimeters in the distal direction).

It should be understood that the reverse configuration is also possible,wherein the isolating diamond is deposited on the tip of the screw andthe rest of the active screw is left uncoated. This configuration hasthe advantage of avoiding high current densities at the screw tip, whichcan locally cause a more intense heat, even more annoying in that it ispoorly thermally transferred by the low conductive surfaces that a pointpresents. In fact, if the tip is isolated and the body of the screw isstripped, heat can be better distributed along the screw and also bettertransferred.

One skilled in the art will appreciate that the present invention can bepracticed by embodiments other than those described herein, which areprovided for purposes of illustration and explanation, and not oflimitation.

What is claimed is:
 1. A heat dissipating housing to be positioned at adistal end of a stimulation/defibrillation lead, the housing comprising:a housing body having a distal contact face at a distal end, an innerwall, and an outer wall; and an anchoring screw, wherein at least aportion of the anchoring screw extends axially from the distal end ofthe housing body; wherein the anchoring screw is configured to securethe distal contact face in contact with tissue of a patient to form athermal bridge between the tissue and the housing body; and wherein thehousing body is electrically insulated and thermally conductive todissipate heat transferred through the thermal bridge to the outer wallof the housing body.
 2. The heat dissipating housing of claim 1, furthercomprising an electrode formed by an electrically conductive portion ofthe anchoring screw.
 3. The heat dissipating housing of claim 2, whereinthe electrically conductive portion of the anchoring screw is a distaltip of the anchoring screw.
 4. The heat dissipating housing of claim 2,wherein the electrically conductive portion of the anchoring screw is aportion of the length of the anchoring screw, not including a distaltip.
 5. The heat dissipating housing of claim 1, having a thermalconductivity of at least 5 W/mK.
 6. The heat dissipating housing ofclaim 1, wherein the housing body comprises an electrically insulatingsurface coating, wherein the electrically insulating surface coating isa thermally conductive material.
 7. The heat dissipating housing ofclaim 1, further comprising a deployment mechanism configured to deploythe anchoring screw out from the housing to a deployed position whereinthe anchoring screw extends axially from the distal end of the housingbody.
 8. The heat dissipating housing of claim 1, further comprisingportions of a thermally conductive material inserted between the turnsof the anchoring screw so as to fill the space existing between theturns.
 9. The heat dissipating housing of claim 1, wherein the innerwall of the housing defines a cavity, and wherein the cavity is filledwith a thermally conductive material.
 10. A heat dissipating housing tobe positioned a distal end of a stimulation/defibrillation lead, thehousing comprising: a housing body having a distal contact face at adistal end, an inner wall, and an outer wall; an electrode positionednear a distal portion of the housing; an anchoring mechanism extendingfrom the distal end of the housing body, to anchor the distal contactface to tissue of a patient; a first and a second thermal bridge fortransferring heat from the tissue to the housing; wherein the firstthermal bridge is formed between the tissue and the distal contact faceof the housing body; and wherein the second thermal bridge is formedbetween the anchoring mechanism and inner wall of the housing body. 11.The heat dissipating housing of claim 10, wherein the first and thesecond thermal bridge provide for transfer of heat to the outer wall ofthe housing body to dissipate the heat to an area surrounding the outerwall.
 12. The heat dissipating housing of claim 10, wherein theanchoring mechanism comprises a helical screw that has an electricallyconductive portion comprising the electrode.
 13. The heat dissipatinghousing of claim 12, wherein the electrically conductive portion is adistal tip of the helical screw.
 14. The heat dissipating housing ofclaim 10, wherein the anchoring mechanism comprises anchoring tinesprojecting outside of the tubular housing.
 15. The heat dissipatinghousing of claim 10, having a thermal conductivity of at least 5 W/mK.16. The heat dissipating housing of claim 10, wherein the housing bodycomprises an electrically insulating surface coating, wherein theelectrically insulating surface coating is a thermally conductivematerial.
 17. The heat dissipating housing of claim 10, furthercomprising a deployment mechanism configured to deploy the anchoringmechanism out from the housing to a deployed position wherein theanchoring mechanism extends from the distal end of the housing body. 18.The heat dissipating housing of claim 12, further comprising portions ofa thermally conductive material inserted between the turns of thehelical screw so as to fill the space existing between the turns. 19.The heat dissipating housing of claim 10, wherein the inner wall of thehousing defines a cavity, and wherein the cavity is filled with athermally conductive material.